In addition to the usual retroviral gag, pol, and env genes, the AIDS retrovirus (HIV) possesses several open reading frames (ORFs) encoded by complex spliced mRNAs. We have concentrated on two of these ORFs, namely the B ORF and the art gene. To determine the functional potential of these transcripts, expressible cDNA plasmids representing viral RNAs were constructed. Complete DNA sequencing of two clones revealed that one of them (pV102) had the 5' leader RNA spliced to a 69 nt central exon which was spliced to a 1300 base 3' exon including the entire B ORF and terminating 12 nucleotides downstream of the poly (A) site in the R sequence of the LTR. The cDNA insert in pV102 lacked the Met codon for the art gene. The other recombinant (pNN37) had intact leader RNA, no central exon and a shorter 3' exon. This clone possessed the entire B ORF but was spliced to the 5' leader 90 bases downstream of the site in pV102. By cell free translation of the T7 transcripts of the cDNA insert in pV102, it was shown to encode two forms of B protein alternatively initiated at two Met codons separated by 20 residues. The functional role(s) of these protein(s) were determining by constructing infectious proviral DNAs containing either a termination codon after the second Met or a frameshift deletion 100 residues from Met. In both cases, there was minimal effect on transient viral replication compared with the standard provirus. The mutant proviruses replicated indistinguishably from wild type proviral DNA in T lymphocytes. Similarly, overexpression of B gene product did not significantly alter the transient expression wild type or B mutant proviruses. To study the function of the art gene product, pV102 was engineered to supply the initial Met Codon. The resultant plasmid programmed the synthesis of art protein both in vitro and in vivo. It was able to transcomplement a mutant provirus carrying a termination codon in the art ORF. Studies are in progress to try and dissect out the different functions of this protein in DNA transfection experiments.